1. Field of the Invention
The present invention relates to techniques for acquiring synchronism to a spread spectrum code in a Global Position Satellite (GPS) system.
2. Description of the Related Art
A transformation of the Global Positioning System (GPS) from a military service with the guarantee of civil use to a true dual service is one of the GPS modernization goals. The transformation includes a new military code (M-code) and an acquisition signal for acquiring the M-code. The modulation method used for the M-code produces a spectrum that exhibits a null at the carrier frequency of the C/A-code. This allows the M-codes to be transmitted along with the existing C/A-codes and P(Y) codes.
The M-code signal is a military data modulation multiplied by a spreading code modulation. The M-code acquisition includes a frequency hop acquisition signal that allows for fast acquisition of the M-code in a jamming environment. A fast acquisition processor uses the frequency hop signal to develop a time estimate that is provided to a direct acquisition processor for direct acquisition of the M-code. The hopper approach to fast acquisition involves transmitting a series of discrete tones from the Satellite Vehicle (SV) at approximately the same signal level as the GPS signal. The actual frequency of these ±5 to ±12 MHz tones are controlled by the M-Code sequence making them secure and unique for any given time. The actual rate of change of these frequencies (hopping) would be at intervals of 1, 5, 10, or 20 milliseconds or the like. By matching the hop signal with all of the possible hop patterns over the period of the known time uncertainty, it is possible to reduce the time error to less than ¼ of the hop interval. To minimize losses caused by the misalignment of the frequency hops between the SV and the receiver, the receiver uses two de-hoppers for each hop interval in the desired time uncertainty spaced at intervals of 0.5, 2.5, 5, or 10 milliseconds. This reduces the residual misalignment of the hops (time error) to one quarter of the hop interval. Currently the Fast Acquisition needs to account for a time uncertainty of ±10 seconds and a frequency uncertainty of ±1600 Hz.
The time estimate produced by the fast acquisition correlator is provided to a direct acquisition processor that actually acquires the M-code. Unfortunately, the ¼ hop interval residual time error left by the fast acquisition processor results in a relatively large time window to be searched by the direct acquisition processor. To minimize acquisition hardware and time, it is desirable to reduce this residual error as much as possible.